If you have backed up (and i do recommend you do a bootable Back Up too with either Carbon Copy Cloner or SuperDuper) it wont be a full install per se. With SL its like the old Archive and Install which will bring the system back to the way it was set up just prior to you doing the install. You will only need to get the 10.6.3 Combo Update and then update all your 3rd party apps you have.  have made it easy for us now

Then once you have done the new install just use Migration Assistant to help you move everything back.

Viola Simple

Cheers

Dont forget to use the Reputation System if someone has helped you out !!!Arguing with a zealot is only slightly easier than tunneling through a mountain with your forehead!!!!!MoTM☆☆☆